It is widely believed that increased concentrations of atmospheric carbon dioxide (CO2) can lead to climate change through global warming effects. The burning of fossil fuels is thought to be chiefly responsible for such atmospheric increases, and governments are beginning to set targets for regulating or reducing anthropogenic carbon dioxide emissions in an attempt to mitigate and reduce such effects.
Liquid fuels, such as gasoline, liquefied petroleum gas (LPG), diesel and aviation fuels, are major sources of atmospheric carbon dioxide emissions. In the main, they are derived from fossil fuels such as crude oil, natural gas and coal. Natural gas and coal, for example, can be converted to syngas through processes such as steam reforming or partial oxidation in which the syngas is subsequently converted into liquid hydrocarbon products by Fischer Tropsch synthesis. Crude oil is typically distilled into various fractions based on different boiling points in a refinery, which can be used as fuels directly, or after further conversion.
One approach for reducing human-related contributions to atmospheric CO2 concentrations is to use biomass as a fuel, or to prepare fuels from a biomass source. Biomass is ultimately produced from atmospheric carbon dioxide through photosynthesis and related processes, and hence any CO2 released on combustion will have been originally derived from the atmosphere. The fuels can therefore be regarded as CO2-neutral.
An example of biomass-derived fuel is biodiesel. One type of biodiesel comprises a blend of regular fossil fuel-derived diesel and a biological oil (bio-oil), typically a plant oil such as rapeseed, sunflower or corn oil. However, use of biological oils directly as a fuel is not always desirable as they can cause engine fouling through coking or polymerisation, and can contaminate the engine lubricant, reducing its effectiveness.
Biological oils are chiefly comprised of fatty acid triglycerides, and they can be converted into hydrocarbons corresponding to the fatty acid hydrocarbon chains by reaction with hydrogen, in a process often referred to as hydrodeoxygenation. An example of such a process is described in U.S. Pat. No. 5,705,722, which relates to the production of hydrocarbons through the hydrogenation of biological oils, and blending the hydrocarbons with diesel fuel.
Another hydrodeoxygenation process has been described by Baldauf & Balfanz in VDE Reports No 1126 (1994) pp 153-168, in which biologically-derived oils can be co-fed with a mineral oil feedstock to a refinery hydrodesulphurisation unit, wherein the mineral oil is hydrodesulphurised and the biological oil hydrodeoxygenated simultaneously to produce a diesel fuel.
However, a problem with the aforementioned hydrodeoxygenation processes is that oxides of carbon (COx) are produced. Typically, these are separated from the product hydrocarbons in the vapour fraction of a flash separator, together with unreacted hydrogen. It is desirable to recycle hydrogen to the reactor to save excessive waste. However, the presence of carbon oxides in the vapour fraction restricts the quantity that can be recycled, as carbon oxides would otherwise accumulate in the reactor at levels which would negatively affect hydrocarbon yields and contribute to catalyst deactivation.